Many industrial processes require the control of the flow of fluids through conduits. In some cases it is desirable to keep a process variable (e.g. pressure, flow, etc.) within an operating range. However, the flow of fluids through a conduit may be subject to disturbances that affect the process variables. To reduce the effect of disturbances, sensors and transmitters collect information about the process variable and its relationship to a set point. A controller may then determine what changes have to be implemented to get the process variable to the desired value. To effect the changes requires a control element such as a control valve. The control valve regulates the flowing fluid (e.g. gas, steam, water, etc.) to correct the changes resulting from the load disturbances.
There are a wide variety of control valves that are in commercial use. These include ball valves; gate valves; check valves; and butterfly valves, among others. Some control valves may include hydraulic actuators (e.g. pilots). These valves respond to changes of pressure or flow to open or close the valve in response to those changes. In some pilot driven control valves the fluid pressure is used to open and close the valve. Other control valves may be responsive to signals generated by independent devices such as flow meters or temperature transmitters.
Some control valves make use of a pilot or pressure amplifier to improve the sensitivity of the control valve to pressure variations. Pilot driven control valves include a pilot valve, a main valve, a pressure conduit, a dome, a piston and a seat. Pressure from the upstream side is provided to the dome often by a small pitot tube. The upstream pressure tries to push the piston open but it is opposed by that same pressure because the pressure is routed around to the dome above the piston. The area of the piston exposed to pressure is larger in the dome than it is on the upstream side; the result is a net sealing force. The pressure from the pitot tube to the dome is routed through the actual control pilot valve. There are many designs but the control pilot is essentially a conventional pressure reducing valve with a stem and a plug that control pressure to the main valve dome. When the pilot valve reaches a set pressure it opens and releases the pressure from the dome. The piston is then free to open and the main valve exhausts the system fluid. In some cases the control pilot opens either to the main valve exhaust pipe or to atmosphere.
The stem of the control valve usually include a packing box (e.g. packing around the valve stem) to prevent leakage of fluids. However, packing box leakage is a common problem and is a large contributor to fugitive emissions in process applications. Several factors exacerbate the problem. For instance, the concentration of hydrogen sulfide in hydrocarbons in some processes is increasing significantly with the shift of production to more challenging oil and gas fields. These compounds negatively impact the performance of the packing box and consequently affect the performance of the control valve.